I used to do the same thing. Then I read that you need to close the racket face more, so I did. However, I still kept floating it. Then I realized that the racket face needs to be just about perpendicular to the ground. It is more the racket path than the angle of the face that produces the slice. Start with the racket face perpendicular to the ground and hold the racket just over your left (assuming your a righty) shoulder and parallel to the ground. Then, as you start your forward swing, the racket face should close as your wrist pronates into the shot. If you hit it with a perpendicular face and it goes into the net or without enough spin, then open the racket face just a little bit until you find the right angle to make it stay low and with spin.